Herbicides and the water quality conundrum
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Herbicidal impact on the health of the Great Barrier Reef (GBR) lagoon came to the forefront in 2009 with the Queensland Government’s Great Barrier Reef Protection Amendment Act 2009 and the concurrent review of diuron by the Australian Pesticides and Veterinary Medicines Authority (APVMA). Subsequently, Federal and Queensland government programs have maintained the spotlight on both freshwater and marine water quality. Ambitious pesticide load reduction targets have been set by the Reef 2050 Plan, as one of the means to improve water quality and the resilience of the GBR ecosystem. Photosystem II (PSII) herbicides in particular are targeted under the plan. Gaining sustained industry practice change is paramount to achieving these targets. Progress is being made, although the challenges remain, both on the practice change level and on the technical knowledge level. Weed management practices with demonstrated environmental benefits include timing spray applications to avoid run-off within the 20–25 days following spraying, incorporation of residual herbicides by non-run-off inducing irrigation or rainfall, switching to strategic and/or banded application of residual herbicides, and avoiding the use of residual herbicides on ratoons where trash blanketing provides sufficient weed suppression. Other farming system improvements such as controlled traffic may reduce the amount of run-off, contributing to reductions in overall herbicide losses. Growers are switching to alternative residual herbicides in response to tighter controls on the PS II herbicides diuron, atrazine, ametryn and hexazinone. Relative risk rankings being developed indicate that alternative herbicides can offer reduced environmental risk.